Mineral oils, polyolefins, and polyethers such as alkylene oxide adducts of alcohols have been used as lubricating base oils of operating oils, gear oils, bearing oils, or other oils. With the recent increase in environmental concerns, lubricating oils have been required to have biodegradability. The above hydrocarbon base oils and polyether base oils are still insufficient in biodegradability. Natural fats and oils such as vegetable oils and derivatives thereof and synthetic ester lubricating base oils have been used as biodegradable base oils (see Patent Literatures 1 and 2).
However, these natural fats and oils and synthetic ester lubricating base oils are easily hydrolyzed when mixed with water, thus exhibiting insufficient stability when lubricating oils are in use. Additionally, these base oils have low water solubility. Leakage of a lubricating oil containing such a low water soluble base oil causes severe environmental pollution due to oil droplets on the water surface. Such a lubricating oil is difficult to wash away or remove. In view of this, polyether base oils with high water solubility and high biodegradability have been used (see Patent Literature 3).